The present invention is directed to a capacitor charging device for a flash, and more particularly, to a capacitor charging device that finishes charging the flash capacitor in a gradual manner.
In accordance with prior art, a capacitor charging device for a flash is usually disposed in a digital camera. It is used to charge the flash capacitor of the digital camera so as to provide electricity to the flash. As shown in FIG. 1, the charging device includes a flash capacitor 10, voltage transforming circuit 20, current mode pulse width modulation (PWM) control circuit 30 and charging control circuit 40. The voltage transforming circuit 20 further includes a transformer T10, diode D10, resistor R100 and metal-oxide-semiconductor field-effect transistor (MOSFET) Q10.
When the charging device functions initially, the current mode PWM control circuit 30 outputs a pulse current I to control the gate-to-source voltage VGS. The charging control circuit 40 controls the pulse width of the current I with a time control capacitor (not shown). Since the capacitance of the time control capacitor is small, the soft start period (i.e. the period for reaching the maximum pulse width of the current I) of the current mode PWM control circuit 30 is very short. Hence, the current mode PWM control circuit 30 can output a pulse current I with maximum pulse width in a very short time.
When the pulse current I is xe2x80x9conxe2x80x9d (i.e. a state with an output current), the gate-to-source voltage VGS of the MOSFET Q10 is positive and results in an increase of the current ID passing through the MOSFET Q10. While the current ID increases, the transformer T10 generates an induced current I0 to charge the flash capacitor 10. When the pulse current I is xe2x80x9coffxe2x80x9d (i.e. a state with no output current), the gate-to-source voltage VGS of the MOSFET Q10 is zero and results in the decrease of the current ID. While the current ID decreases, due to the tremendous reverse resistance of the diode D10, the induced current I0 also reduces to zero. Thereby, the flash capacitor 10 can be charged by the induced current ID in the on-off circle of the pulse current I.
However, since the soft start period of the conventional charging device is very short, the induced current ID reaches its maximum value in a very short time. Hence, the conventional charging device easily damages the external circuit, flash capacitor 10 and the rechargeable battery of the digital camera so that the life of the digital camera is decreased. Further, the conventional current mode PWM control circuit 30 is expensive (comparing with the voltage mode PWM control circuit) and makes the digital cameral costly.
Accordingly, as discussed above, the conventional charging device has some drawbacks that could be improved. The present invention aims to resolve the drawbacks in the prior art.
An objective of the present invention is to provide a capacitor charging device for a flash that can charge the flash capacitor completely within the soft start period so as to prevent the induced current from increasing abruptly. Thereby, the prevent invention can reduce the damage of the external circuit, flash capacitor and rechargeable battery and increase the life of the digital camera.
Another objective of the present invention is to provide a capacitor charging device for a flash that uses a voltage mode PWM control circuit to replace the current mode PWM control circuit to reduce the cost of the digital camera.
Still another objective of the present invention is to provide a capacitor charging device for a flash that gradually increases the pulse width of the pulse voltage output from the voltage mode PWM control circuit to prevent instantaneously large current. Still another objective of the present invention is to provide a capacitor charging device for a flash that use a time control capacitor ranging from 3.3 uF to 22 uF to control the length of the soft start period.
For reaching the objective above, the present invention provides a capacitor charging device for a flash that includes a flash capacitor, voltage transforming circuit, voltage mode PWM control circuit and charging control circuit. The charging control circuit has a time control capacitor ranging from 3.3 uF to 22 uF to control the voltage mode PWM control circuit to increase the pulse width of the pulse voltage gradually within the soft start period. Then, by using the pulse voltage, the voltage mode PWM control circuit can drive the voltage transforming circuit to provide an induced current that increases gradually to charge the flash capacitor. In this manner, the present invention can reduce the damage of the external circuit, flash capacitor and rechargeable battery, and charge the flash capacitor completely within the soft start period.
Numerous additional features, benefits and details of the present invention are described in the detailed description, which follows.